1. Field of the Invention
The present invention relates to a friction member provided in an ultrasonic motor for transmitting a vibration of the ultrasonic motor to a movable body. The present invention relates to a guide apparatus in which a ultrasonic motor drives a movable body linearly or rotationally.
2. Prior Art
It is features of ultrasonic motors as power sources for controlling and driving use that a minimum amplitude by vibration is small in an order of nanometer, allowing for precise positioning with high revolution. The ultrasonic motors can provide a large driving force due to friction drive with a compact size. The ultrasonic motors have been conventionally utilized in rotation control systems including lens zoom mechanism for cameras and vibration alarms for wrist watches. In recent years the ultrasonic motors are applied to linear control systems.
The ultrasonic motors can achieve high positioning accuracy during movement of the movable body so that they are desirable for guide apparatuses in precise machining tools or precise measuring apparatuses, and in pattern exposure apparatuses in semiconductor manufacturing systems or the like.
FIG. 8 shows an example of a conventional guide apparatus using an ultrasonic motor for controlling a linear movement of a movable body. This guide apparatus is provided with a linear guide member 12 such as, for example, a pair of cross roller guides, on a base substrate 11, which member guides a stage 13 as a movable body in a linear direction.
A drive transmitting member 14 is fixed to the stage 13 of the guide apparatus on one side thereof in parallel to the guide members 12, and a linear scale 16a is fixed on another side in parallel to the drive transmitting member 14. A measurement head 56b is provided at a position facing to the linear scale 16a on the base substrate 11, making up a position detecting means 16
An ultrasonic motor 15 is fixed to a position opposing to the drive transmitting member 14 on the base substrate 11, which has a friction member 15a arranged with a contact face in a leading end portion thereof, the contact face being in perpendicular contact with a contact face of the drive transmitting member 14. The stage 13 can be guided by the guide members 12 and 12 so as to move and stop via the drive transmitting member 14 in accordance with an ultrasonic vibration of the friction member 15a. 
In this example, a case 15d contains the ultrasonic motor 15 therein, both sides of which are held by some springs 15b, with a spring 15c being interposed between a rear portion of the case 15d and the ultrasonic motor 15. A friction member 15a is pressed against the drive transmitting member 14 of the stage 13 so as to be brought into contact therewith.
The ultrasonic motor 15 is shown in FIGS. 10A and 10B, which is made up of a piezoelectric driving portion 15e for generating an elliptic vibration, with the friction member 15a fixed to, and projected from, the piezoelectric driving portion 15e. The friction member 15a is made of glass or ceramic.
In FIGS. 10A and 10B, the piezoelectric driving portion 15e includes a piezoelectric ceramic plate 15f, four electrode films 15g, 15h, 15i and 15j separated into pieces on one major surface of the ceramic plate 15f on one main surface thereof, and a common electrode film formed on substantially all of another major surface of the ceramic plate 15f, and the electrode film 15g and the electrode film 15i positioned at opposing corners are connected by wire, and the electrode film 15h and the electrode film 15j positioned at opposing corners are connected by wire. The electrode film 15i generates a vertical vibration and a horizontal vibration in the piezoelectric ceramic plate 15f by grounding the common electrode film and applying an electric voltage having 90 degrees different in phase from the electrode film 15j. These two mode vibrations are combined with each other so as to elliptically move the friction member 15a and move the stage contact therewith by a narrow step.
Further, in FIG. 8, the driving control portion 10 executes a feedback control for the stage 13, for example, by executing an arithmetic processing necessary for a PID control and outputting a command signal to the ultrasonic motor 15 in correspondence to a deviation between a position information output from the position detecting means 16 in accordance with the movement of the stage 13, and a reference position information on the basis of a preset movement profile of the stage 13, and outputting a command signal to the ultrasonic motor 15. In conventional, P term, I term and D term corresponding to control parameters for executing the PID arithmetic processing are experimentally determined by trial and error before a driving operation is performed, so that a position deviation and a positioning accuracy during the movement of the stage 13 satisfy standards.
In this case, in the guide apparatus used in the working machine for the precision processing, the precision measuring apparatus or the drawing exposure apparatus in the semiconductor manufacturing process, a small position deviation in a sub micron order and a positioning with high accuracy are required during the movement with high accuracy, and a long life time and a high reliability for a long period of time are desired.
However, since the ultrasonic motor 15 is driven by utilizing a friction, a slip tends to be generated between the friction member 15a of the ultrasonic motor 15 and the drive transmitting member 14 of the stage 13. The generation of slip makes positioning of the stage 13 with high accuracy hard, due to accumulation of the slip. Further, the slip changes a contact state between the friction member and the drive transmitting member, and makes any one of them abnormally wear out.
In view of a material of the friction member 15a, since the friction member 15a of the conventional ultrasonic motor 15 slides in a state of being pressed to the drive transmitting member 14, it is necessary to form from a material which is excellent in an abrasion resistance. In conventional, there are the friction member formed by a glass material such as a silica glass, a soda glass or the like, or a ceramic based sinter such as an alumina based sinter having 99.5% by weight or more alumina content, a zirconia based sinter, a silicon carbide based sinter or the like (see Japanese Patent Publication No. 7-273384)
The friction member 15a of the ultrasonic motor 15 employing the glass material such as the silica glass, the soda glass or the like has a small fracture toughness value, and has a risk that a chip or a fracture is generated when a crack is generated. Accordingly, when moving a heavy stage 13 by the ultrasonic motor 15 provided with the friction member 15a made of the glass, there is a case that the chip or the fracture is generated in the friction member 15a by a stress applied at a time of frictionally driving together with the drive transmitting member 14, and it is necessary to stop the guide apparatus every time when the chip or the fracture is generated.
Further, when driving the ultrasonic motor 15 at a high speed, there is a case that a friction heat generated with respect to the drive transmitting member 14 becomes a high temperature which is higher than a softening point of the glass material forming the friction member 15a, and it is hard to apply to a high speed driving.
On the other hand, the friction member 15a of the ultrasonic motor 15 using the ceramic based sinter made of the alumina, the zirconia or the silicon carbide has an advantage that the friction member is hard to be broken in comparison with the glass member. However, in the silicon carbide based sinter, in accordance with a self-lubricating effect, a friction coefficient with the contact face 14a of the drive transmitting member 14 is small, and a slip is generated by driving the ultrasonic motor 15 at a high velocity, so that it is impossible to move the stage 13 at a high speed.
Since a Vickers hardness of the zirconia based sinter is about 12 GPa which is small in comparison with the other ceramic sintered bodies, the zirconia based sinter tends to wear out for a comparatively short period of time due to the friction driving with the drive transmitting member 14. In addition, when driving the ultrasonic motor 15 at a high velocity, there is a case that a friction heat with the drive transmitting member 14 goes over 100° C. Since the zirconia based sintor is generally brittle against heat, there is a risk that an abrasion make progress in correspondence to deterioration in a mechanical characteristic or the like.
The high-alumina based sinter, for example, containing 99.5% by weight or more alumina content, has a very high hardness, however, the high-alumina based sinter is hard to be sintered and requires a high temperature sintering. A normally used alumina based sinter contains about 1 to 2% by weight of sintering aid such as calcia (including calcium oxide), magnesia, silica or the like. A crack is generated in a boundary layer constituted by the sintering aid mentioned above due to a stress applied at a time of frictionally driving with the drive transmitting member 14, and a berry drop of alumina grains is generated. If the grain drop is generated, there is a problem that a scratch is generated in the drive transmitting member 14 by an edge of a recess portion thereof, and the drop grains are bitten into the drive transmitting member 14 so as to scrape and wear out the friction member 15a and the drive transmitting member 14. Since the contact state is changed if the drop grains are bitten into the drive transmitting member 14, there is a risk that a bad influence is applied to a moving characteristic, a positioning accuracy and the like of the stage 13, and a life time of the apparatus is shortened.
In view of a mechanism of the guide apparatus, there is proposed a guide apparatus in which monitoring a slip between the ultrasonic motor 15 and the stage 13 is reflected in a control. Japanese Unexamined Patent Publication No. 2000-308939 discloses using a preload adjusting portion 21 such as a piezoelectric actuator or the like which is expanded or contracted in accordance with an electric voltage application in place of a spring (55c in FIG. 5) applying a preload to the ultrasonic motor 15, as shown in FIG. 9. There is disclosed that this guide apparatus is provided with a non-contact type measuring means 5 such as a laser displacement measuring machine or the like so as to measure position data including a displacement, a velocity and an acceleration of the friction member 15a during the operation of the ultrasonic motor 15, and a slip distance measuring portion 23 calculates a slip distance between the ultrasonic motor 15 and the stage 13 on the basis of the position data of the friction member output from the non-contact type measuring means 5 and the position data of the stage output from the position detecting means 16. This calculated value is used for driving the preload adjusting portion 21 in correspondence to the slip distance so as to adjust a pressing force of the ultrasonic motor 15 applied to the stage 13.
The conventional guide apparatus can forecast a risk of abrasion increased by following up the slip state between the ultrasonic motor 15 and the stage 13, however, since a degree of abrasion is largely different in the uniform slip distance if the driving speed of the ultrasonic motor 15, the preload or the weight of the stage is different, it is impossible to forecast how much the abrasion is made progress in accordance with the driving condition in the case that only the slip distance is monitored.
In particular, if the driving is frequently continued under such a severe condition as the stage 13 is driven at a high speed or a high acceleration, there is a case that the friction member 15a of the ultrasonic motor 15 or the drive transmitting member 14 of the stage 13 is abnormally worn out. The abnormal abrasion makes the guide apparatus break down and makes the life time of the guide apparatus short beyond expectation, however, it is hard to forecast the life time of the guide apparatus.
Accordingly, in the guide apparatus used for the precise working machine, the precise measuring apparatus and the drawing exposure apparatus in which the high position accuracy of the stage 13 is required in the middle of the driving, a further high reliability is required.
In the conventional guide apparatus, since it is necessary that the non-contact type measuring means is always mounted to the apparatus for measuring the slip distance, there is a problem that the structure of the apparatus is made complicate and large size. Further, there is accordingly a problem in view of a cost.
Accordingly, in order to measure the slip in the precise measuring apparatus and the drawing exposure apparatus in which a small occupied space is required, it becomes hard to design and there is a limitation in design.